The invention relates to new and useful improvements in the manufacturing of printed wiring boards, and more particularly to installation of surface mount components on printed wiring boards, and most particularly to soldering such surface mount components on to printed wiring boards.
Surface mounting of electronic components on printed wiring boards has become the prevalent mounting method for boards used in high quality, high density electronic equipment. This method not only improves the reliability and performance of such boards but also increases the efficiency of the manufacturing process. Surface mount technology is explained in an article in OST magazine, October, 1987, on page 15 et seq., which is incorporated herein by reference.
In the present practice, surface mount components are often soldered to printed wiring boards using the vapor phase technique. In this technique a solder paste is applied to the printed wiring board pads and, a component is positioned and held in place atop the pads by the paste. The board is then passed through or placed in a chamber containing the vapor of a liquid which has a boiling temperature higher than the melting temperature of the solder. The solder is thus melted, and the printed wiring board is then passed or placed into a cooling chamber where the solder hardens into its final state.
The vapor phase soldering process can result in unwanted component shifting due to solder movement. This shifting in component position can result in defective assemblies which must be repaired by costly manual methods, and it increases tuning time of component placement sensitive designs. Shifting can be minimized by containment of solder in the immediate pad termination area. Containment of solder thus results in printed wiring board assemblies approaching the production goals of zero defect soldering and minimized electrical adjustment time.
The present approaches to solder containment vary depending upon the board material. On polymide and glass boards, solder containment involves minimization of the size of the pad termination area and the width of the signal lines attached to the pads. In addition, solder mask material may also be applied over large areas of the component side of the printed wiring board and circuit traces primarily to prevent electrical shorting of circuit traces from conductor to conductor or component to component. On TEFLON.TM. boards, however, these approaches may have limited usefulness for several reasons. First, radio frequency printed wiring boards for use in the VHF/UHF ranges typically have circuit line widths larger than at lower signal frequencies in order to maintain the required characteristic impedance. Second, the solder mask material is typically not applied to bare TEFLON.TM. itself, and other board materials having similar characteristics, since its adhesion to the TEFLON.TM. material is poor and would tend to flake off. Finally, any solder mask material bridging RF circuit traces would effectively increase the dielectric constant of the TEFLON.TM. board material resulting in more electrical losses and degraded frequency range.
The generally accepted practice for solder containment in TEFLON.TM. designs is to fabricate a "neck", into the circuit trace on the printed wiring board. However, the circuit trace necking approach results in degraded electrical performance due to the unwanted parasitic effects created by induced resistance, inductance and capacitance. In addition, the circuit trace necking approach does not entirely eliminate the component shift problem.
The approach of the present invention involves application of a solder mask over bare copper. Solder mask is commonly placed on the bottom side of a printed wiring board to prevent solder bridging during wave soldering. However, there are no known uses of a solder mask on the top side of a TEFLON.TM. printed wiring board for the purpose of preventing solder movement to ensure proper placement of surface mount components.
It is, therefore, an object of the present invention to improve the soldering of surface mount components to printed wiring boards such that the goal of zero defects is more nearly attained.
It is further an object of the present invention to eliminate the shifting of surface mount components in the vapor phase soldering process.
It is yet another obJect of the present invention to decrease the cost of manufacturing printed wiring board assemblies containing surface mount components.
It is still another object of the present invention to decrease the cost of testing printed wiring board assemblies containing tunable or placement sensitive surface mount components.